SWITCHBOARDS
CB 935, CBS 364 & CBS 484

In the Technical Instructions XXIII (1924) these switchboards are referred to as Switchboard B.E., C.B. Cordless, followed by the amount of lines and extensions.

i.e. Switchboard, B.E., C.B., Cordless 1+3.

A cross reference is given in Table 1.

In Technical Instructions XXIII (1930) the switchboards names have changed to the explanatory diagram number, followed by the amount of lines and extensions.

The original cordless switchboard was introduced circa 1907.  These were negative clearing types and had a dolls eye indicator associated with every connecting link.  These indicators were fitted directly above the exchange and extension indicators.  These indicators were made redundant in 1919 by the change to positive clearing.  The negative clearing switchboards were made as 1+3, 2+4 and 3+9.

Click here for Cordless switchboards with negative clearing.

Referred to in the 1928 Rate Book.

These switchboards use a Generator No. 26A with a Handle, Generator No. 15.

All these switchboard require a power supply of 12 volts DC but will work up to 20 volts DC.

The fuse in all these switchboards is a Fuse No. 35/1.

Terminal Strip
Battery + to terminal 21.  Terminal 21 is strapped to 13, 11 (RET 11) and 9 (8uf).
Battery - to terminal 22.  Terminal 22 is strapped to 20 and the fuse.  The fuse output is strapped to terminals 16, 14, 12 and 10.

Key Handle Colours
Operator - Grey
Exchange - Grey
Exchange Hold - Grey
Extension - Grey
Ring - Grey
Night Service - Red

Drawings
CB 935 - 9615
CB 935 - 96126
CB 935 - 9756 Mk 5
CB 935 - 9757 Mk 5
CB 935 - 9758 Mk 5
CBS 364 - 60728 (Mark 2) 1+3
CBS 364 - 60729 (Mark 2) 2+4
CBS 364 - 60730 (Mark 2) 3+9
CBS 484 - 63406 (Mark 4) 1+3
CBS 484 - 60260 (Mark 2) 1+3
CBS 484 - 63407 (Mark 3) 2+4
CBS 484 - 60261 (Mark 2) 2+4
CBS 484 - 60262 (Mark 2) 3+9
CBS 484 - 63408 (Mark 3) 3+9

Diagrams
CB 934 - Superseded by the LD diagrams
LD 90 (1+3)
LD 91 (2+4)
LD 92 (3+9)
LD 147 - Wiring for all types
N 934 (CBS Schematic)
N 935 (CB Schematic)
N 936 (Schematic - superseded by N 935))

Specification - S78C

Ericsson supplied some of these with the 1+3, their model N105.

Click here are additional large size quality pictures

ENGINEERING INSTRUCTIONS
TELEPHONES
P.B.X.s
B 1050
Issue 1, 30.8.38

PRIVATE MANUAL BRANCH EXCHANGES
Details of Cordless Switchboards

1. General
This Instruction describes the details of, and the facilities provided by "Switchboards, CB 935, CBS 364 and CBS 484".  These switchboards are of the cordless, table type and are issued in four sizes.  They are for use on the systems shown below.

TABLE 1

2. Equipment, Capacity, Dimensions and Weight
The Table below gives the capacity and equipment of the four sizes of switchboards.  Each system type includes all four sizes.

DIMENSIONS AND WEIGHTS

3. Wiring
The wiring is complete for the capacities quoted in Table below:-

EQUIPMENT AND CAPACITY OF CORDLESS SWITCHBOARDS

NOTE  On the 3+7 (12) switchboards, plain face-plates on wooden dummy indicators are fitted on extension positions 8 and 9, and key spaces - of the same dimensions as the other keys - are fitted in the corresponding positions on the key panel.

Switchboard CBS 484 1+3 (4)		Switchboard CBS 364 2+4 (6)
Switchboard CB 935 3+7 (12)

The pictures above were taken in 1924

4. Diagrams

CB 935
Switchboard Circuits (explanatory) - N936
Switchboard Circuits (wiring details) - CB 934 and N935
Switchboard Circuits (wiring details Mk 5) - LD 90

CBS 364
Switchboard Circuits (explanatory) - N908
Switchboard Circuits (wiring details) - CBS 409 and 364
Switchboard Circuits (wiring details Mk 5) - LD 91

CBS 484
Switchboard Circuits (explanatory) - N951
Switchboard Circuits (wiring details) - LD 46 - 48 and C.B.S. 484
Switchboard Circuits (wiring details Mk 5) - LD 92

5. Transmission data
The maximum permissible resistance of exchange line, plus extension, is given in the following E.I.s:-
TRANSMISSION, Telephone, B 3503 - C.B. manual and automatic areas.
TRANSMISSION, Telephone, B 3505 - L.B. telephones in C.B. (manual) and automatic areas.
TRANSMISSION, Telephone, B 3550 - L.B. exchange areas.
TRANSMISSION, Telephone, B 3590 - Inter-switchboard extensions.

6. Design and equipment
These switchboards are designed to stand on a table or shelf.  The front of the board is hinged at the bottom and the to removable, to afford access to the interior of the board.  When the front of the board is lowered for inspection, the weight of the front panel is taken by strips of webbing fixed to the sides of the boards.  The handle of the generator projects on the right hand side, and the wiring is led into the switchboard through a hole in the left-hand side.  Different colour key handles are used and these are defined in the N diagram associated with each model.

7. Night alarm
A buzzer is included in the equipment of the switchboard, to indicate when either a calling or clearing signal is received.

8. Night service
On certain of the connecting circuits, night-switching keys are provided, the operation of which enables the corresponding connecting circuit to be used for connecting any extension to any exchange line, for night service.  The number of night-switching keys is limited to the exchange line capacity of the board.  The operation of any night-switching key will disconnect the night alarm from all the exchange-line indicators and also the supervisory relay on the connecting circuit concerned.

9. Ringing
Ringing current for local circuits is provided by a hand generator fitted in the switchboard.

10. Operator's telephone
A standard instrument without bell-set, as appropriate for direct exchange lines in the same area, is used for operating the switchboard.

11. Designation strip
A transparent-fronted designation strip is fitted between the indicators and keys.  The circuit designations are marked on a paper strip placed under the transparent strip.

12. Termination of exchange and extension lines on switchboards

(a) Exchange lines
These are terminated on drop-type indicators, which are operated by ringing current and must be restored by hand.  The indicators fitted in the latest issues of these switchboards are "Indicators No. 3700A", which are not operated by dial impulses, as were "Indicators No. 2200A" previously used, see B 5901 and B 5903.  Each exchange line is connected to keys arranged in a column vertically below the exchange-line indicators.  The lowermost key in each column is used as a HOLD key, except on "Switchboards, CBS 364" on which a HOLD key is unnecessary, see paragraph 14.

(b) Extensions
These are terminated on doll's eye indicators, which restore automatically when the extension clears.  Each extension is connected to keys arranged in a column beneath the indicator similarly to the exchange lines-the lowermost key is used as a RING key and is individual to its associated extension.

(c) Operating telephone
This is similarly connected to keys situated on the right-hand side of the switchboard.

13. Connecting circuits
The keys in the other rows are commoned horizontally to form connecting circuits, which are separate for each row, consequently the movement in the same direction of any two keys in the same row connects together the two lines concerned.  The keys lock in all positions, with the exception of the RING positions on the lower half of the bottom row of extension-line keys.

14. Hold
A special holding key or circuit is not provided on "Switchboard, CBS 364", as the operation of any exchange-line key disconnects the clearing earth from the exchange line.  Any exchange line can, therefore, be held when required, by throwing the corresponding exchange-line key of a disengaged connecting circuit.

15. Supervisory signals
The switchboards are of the positive clearing type, the extension calling signals being also used as supervisory signals when an extension is connected.  Through-clearing on exchange connexions is provided; on an exchange-to-extension connexion, the replacement of the extension receiver causes the clearing signal to be given on the extension indicator at the P.B.X. and, simultaneously, at the main exchange.  Individual supervision is not provided on local connexions; a double-clear is given when the receiver is replaced at one of the extensions on "Switchboard, CBS 364" while on "Switchboards, CB 935 or CBS 484" clearing signals are not given until the receivers are replaced at both extensions.

16. Clearing
In C.B.S. No. 1 areas, if the main-exchange operator clears the connexion before it is cleared at the P.B.X., the clearing signal at the P.B.X. is released.  This is a recognised disability and any case in which complaint is made by a subscriber, should be referred to the E.-in-C. (S1), when suitable instructions will be issued.

17. Calling-in
On "Switchboards, CB 935 and CBS 484" the HOLD key may be kept in the operated position to enable the extension user to "call-in" the P.B.X. operator during the progress of an exchange call without breaking down the exchange connexion.  The subscriber's attention, however, should not be drawn to this facility, unless "calling-in" facilities have been specifically demanded.

18. Prohibition of exchange service on private wires
The circuit arrangements for a private wire prevent connexion to an exchange line.

19. Power supply data
On Switchboards, CBS 364 current is not drawn from the P.B.X. bus-bars during conversation, but only when a calling or clearing signal is given.  On "Switchboards, CB 935", however, current is drawn during conversation on local connexions, as well as when calling or clearing signals are given, but not during conversation on exchange connexions.  On "Switchboards, CBS 484" current is drawn during conversation on both exchange and local connexions, but this current is considerably less than that drawn when calling or clearing signal are given.

Operation

Each row of keys is two connecting links.  One up and one down, except for the bottom row which only has one connecting link which is up.  The ring key is the down promotion on the bottom row.

A call be it internal or external is connected by the use of any one of the connecting links.  To answer an extension a corresponding key is operated and also the Operators key on the same connecting link.  To connect the extension to an exchange line, an exchange line key on the same connecting link is operated.

To call an extension the extension ring key must be depressed whilst at the same time operating the hand generator.

Whilst extending an exchange call, the call can be placed in hold so the caller cannot hear the conversation with the extension.

Additional Information

The CB935 (1+3) Mark 4 is made by British Ericsson - their model N105 (Diagram N14964 - Dated 1932).

Switchboards to GPO Diagram CB935E are made by British Ericsson - their Diagram Number N16549 - dated 1934.

The CB935 (3+9) and (3+12) are made by British Ericsson - their Diagram Number N16551 - Dated 1933.

There was a 3+7(10) but this was superseded by the 3+7(12) before 1928.

Additional Pictures

Switchboard CB935 (1+3) - Picture taken in 1957
 

Switchboard CB935 (2+4) - Picture taken in 1957

Switchboard CB935 (2+4) - Looking down from the top - lid off
Terminal strip on left - Capacitors and Induction coils at the rear
A buzzer is fitted to the right

Switchboard CB935 (2+4) - Top removed and front swung down on hinges
Hand generator is to the left

 

Switchboard CB935 (3+9) - Picture taken in 1957
 

Last revised: November 14, 2023